Chain saw



mywfi. A. N. BLum 2,295,240

CHAIN SAW Filed Nov. 14, 1939' 4 Sheets-Sheet 1 Sept; 22, 1942. A N. BLUM CHAIN SAW 4 Sheets-Sheet 2 Filed Nov. 14, 1939 A. N. BLUM Sept. 22', 1942.

CHAIN SAW Filed Nov. 14, 1959 4 Sheets-Sheet 3 lllrlllrll ZZZ Sept. 22, 1942. L 2,296,240

' CHAIN SAW File d Nov. 14, 1939 4 Sheets-sheaf 4 a... jggggnam rams see zaia a 'um-rea sm' 'ss i Arthur N. Application This invention relates to improvements in chain saws.

One object of the invention; is to provide a novel semi-automatic adjustment means for accurately tensioning the saw chain. 1

Another object of the invention is to provid novel and-improved means for lubricating the The invention'further resides in certain novel structural details and arrangements hereinafter ment of the tension-adjusting means;

Figs. 5 and 8 are detached views in perspective of elements of the tensioning device;

-' Fig. 7 is a sectional view on the line 'I1, Fig. 2; Fig. 8 is a sectional view on the line 8-4, Fig. 2; Fig. 9 is a fragmentary view in. perspective showing a detail of the lubricating device;

Fig. 10 is a view in perspective of one of the elements of the lubricating device;

F18. 11 is a fragmentary sectional view illustrating a modification within the scope of the invention; Fig. 12 is a view in perspective showing a meth- 0d of guiding the saw'in a framing cut;

Fla. 13 1s a fragmentary view taken from the position indicated as llil in Fig. 12;

Fig. 14 is a reduced elevational view from th handle end of the saw in Fig. 12, and y Fig. 15 is a plan view illustrating a means for supporting and guid the saw in a transverse out. r

With reference to Fig. l, a saw made'in accordance with my invention comprises a base frame I, a guide rail 2 which is supported at one end in the base frame, and a strain head I which is supported at the outer end of the rail 2. The main frame i comprises in the present instance. a housing. 4 for an electric motor, and a gear case 5 which also houses the driving sprocket for the toothed chain 5. The base frame also comprises two handles 1, I by means of which the operator supports and manipulates the saw in well known manner, and one of the handles carries an electric switch device 8 for controlling the electric motor which is connected through the gearing with the chain sprocket.

10mm saw mm. Pa. November 14, 19:9, serial as seam is calm. (cl. 14:42)

'lheraili consistsof see-Figs. I, s ando, and an intervening illler plate II. the filler plate being of lesser breadth than the two side plates-land ll so as to provide at top and bottom of the rail longitudinal grooves" and I4 within whichthetoothedchain iisguidedinwellknownmannerandasshown in'Figs.7and8.'lhep1ates a, n and lfare. V I

secured together in the present instance by means of rivets ll shown in Figs. 21nd '1. Except in certain details hereinafter described. the rail 1 constitutes no part of the present invention.

Thestrainheadl.shownin1"igs.2and7,is formed to provide at the top a reservoir It for lubricating oil, and at the bottom a housing for an adjustable guide roll I! for the chain I, the said roll preferably taking the form of a sprocket asshown. The outer end of the rail 1 fits into a slot between the two side portions II and II of thehead and is clamped solidly therebetween I by means of a screw fl which passes through the rail as showninl'ig. 2. The headisflu'ther held in fixed position with respect to the rail 1 by means of dowel plug II, hereinafter described, and also a tie rod 22 which extends above and parallel to the guide rail 2 and is secured at its opposite ends to the base frame i and to the strain head 3, as shown in Pig. 1. The primary function of the rod 22, however. is to act'as a guard for the upper run of the saw chain U, and as a safety device in event of breakage of the chain inoperation.

The sprocket I1 is Journaled on a shaft 23 between the bifurcations 24, 24 of a member II which is mounted for sliding adjustment in the head I. The member II, see Fig. 5, has a cylindrical extremity 26 which is slidsblyfitted into the cylindrical interior ofa tubular member 2! which is secured by screws 28 to the outer end of the head 8. The extension 26 of the member 25 is provided with alongitudinal slot or keyway 29, and this slot receives a feather 3| which, as shown in Fig. 2, is clamped between the body of the head I and the flange II of the. member 21. The body of the head I is slotted at 33 in accurate alignment with the slot between the side members I! and I! of the head wherein, as

previously described, the end of the guide plate PATENT or-Pica The cylindrical portion 26 of the member 25 is provided with an axially extending threaded bore 34 which in assembly and as shown in Fig. 2 receives the correspondingly threaded end of an adjusting rod 35. This rod extends outwardly through the cylindrical member 21 and has at its outer end a washer 36 which forms a seat for a coiled spring 31. The inner end of the spring 31 seats against a thrust bearing 38 in the bottom of a recessed sleeve member 39, this member 39 being supported in a countersunk recess in the outer end of the member 21, and being pressed by the spring 31 against the shoulder 4| in the bottom of said recess. The inner end of the sleeve member 39 hasa ratchet connection, as shown at 42 in Fig. 4, with a collar 43 of externally cylindrical form which occupies the interior of the member 21 and which, with the member 39, is free to turn in the latter. This collar 43 is provided with a rectangular bore 44 which receives the correspondingly rectangularly formed outer end portion 45 of the adjusting rod 35 so that it turns with the latter but is free to be displaced longitudinally to it.

As previously stated, the outer end of the collar 43 has a ratchet engagement with the sleeve 39. Behind the collar 43, the adjusting rod 35 carries a pair of stop nuts 46 and a sleeve 46a, said sleeve forming a seat for a split washer 40 which bears also against the end of the collar 43. The washer 40, acting as a spring, exerts pressure to force the said collar resiliently against the opposed ratchet face of the sleeve 39, and performs the added function of a lock washer precluding loosening and displacement of the stop nuts 46 on the adjusting rod. Obviously, the washer 40 can, without functional change, be substituted by a spring 40a, as illustrated in Fig. 11. The washer 40 or spring 40a constitutes an important functional element of the device as hereinafter more fully set forth.

When the sleeve 39, which is formed at its outer end with a hand-grip, as illustrated, is turned in a counterclockwise direction, as viewed in Fig. 1, the teeth of the ratchet device 42 functlon to positively interlock the sleeve with the collar 43, with the result that the rod 35 is backed away in the threaded bore 34 of the member 25, .thereby relieving the tension on the chain 6. To increase the tension on the chain, the sleeve 39 is turned in the opposite or clockwise direction; and in this case, the ratchet 42, by reason of its form, as shown in Fig. 4, will not positively couple the sleeve to the collar 43, the connection between the collar and the sleeve in this case being dependent entirely upon interfriction of the cam-like surfaces of the interengaged teeth of the ratchet 42 asregulated by the pressure of the spring 31 and secondarily of the washer 40. As the rod 35 advances into the member 26, the tension on the chain 6 increases, and when this tension reaches a predetermined point the teeth of the sleeve 39 will slip on the opposing teeth of the collar 43 so that in effect the sleeve will become uncoupled from the adjusting rod 35 and is no longer effective to turn the rod in that direction. It is apparent that the actual amount of tension in the spring 31, which will always exceed that of the spring or washer 40, the latter being only intended to keep the two mating parts of the ratchet in constant positive engagement, may be regulated by adjustment of the collar 43 on the rod 35 through the medium of the nuts 46, so that the spring 31 may be set to afford an exact amount of tension in the chain 6 beyond which the ratchet 42 will slip or in absence of spring washer 40 be entirely disengaged, as described above, to prevent any further tightening of the adjusting rod 35.

It will be noted that in the aforedescribed construction, the adjusting screw assembly, is made to constitute a self-contained unit which may be removed intact from the saw structure. In the operation of the saw, it is important that this unit be maintained securely in position so as to preclude backing out of the adjusting screw, since any material release or loosening of the tension under which the toothed chain is held in operation will result inevitably in rupture of the chain. It is a primaryfunction of the spring or washer 40 to aid in maintaining the adjusting rod or screw in position and to prevent any movement of this screw due to oscillations during cutting which might modify the tension of the toothed chain.

It will be noted that in assembly and aside from the normal frictional contact between the interengaged threads of the adjusting rod 35 and the member 26, the sole factor precluding rotary movement in the housing of the adjusting rod and of the adjusting rod assembl as a whole is the frictional engagement between the sleeve 39 and the shoulder 4| against which the sleeve is pressed by action of the spring 31. In the normal operation of the saw a considerable longitudinal vibration is set up in the member 25 and in the adjusting screw 35, and it is apparent that in the absence of the washer 40 or spring 40a at the back of the collar 43, this vibration would have a tendency both to unseat the sleeve 39 from the shoulder 4| and to permit the collar 43 to retract from its normal frictional engagement with the said sleeve 39. Any unseating of the sleeve 39 or loss of solid frictional contact between the sleeve and the shoulder 4| would obviously permit the adjusting rod assembly to turn, with consequent loss of tension in the toothed chain and eventual breakage of the latter. Release of the solid frictional engagement between the collar 43 and the sleeve 39 would permit angular movement of the adjusting rod 35 with the same result. By insertion of resilient means between the lock nuts 46 and the collar 43, it is apparent that there may be a movement of the adjusting rod 35 to the left, as viewed in Fig. 2, without unseating the sleeve 39 from the shoulder 4|, the relative movement between the adjusting rod and the collar 43 being taken up by compression of the resilient means. It will be apparent also that any movement of the adjusting rod in the opposite direction will not result in a release of the normal engagement between the collar 43 and the sleeve 39, the resilient means in this instance functioning by expansion to maintain the collar 43 in its normal position of engagement with the sleeve 39. Also the sharp edges of the washer 40 preclude any relative movement between the collars 43 and 46a. and help to maintain the various elements of the assembly in their proper relative positions angularly of the adjusting rod 35. It is important, therefore, that the nuts 46 be so placed that the washer 40 or other resilient means shall not under normal conditions be pressed entirely flat between the collars 43 and 46a, and that it shall normally be under partial compression. If a spring is used, as shown in Fig. 11, in lieu of the resilient lock washer, the terminal ends of the spring should preferably present to the surfaces against which of the casing I they seat the sharp locking edges which are characteristic of resilient washers of the type shown in Fig. 2.

Another desirable characteristic of the construction described above resides in the fact that after the lock nuts 46 have been set in proper position upon the adjusting rod to afford the required tension in the toothed chain and to establish the elements of the adjusting rod assembly in their proper relative positions as described above, there need be no further adjustment of the lock nuts under normal conditions, and they may, therefore, be established in this position by tamper-proof means, such, for example, as by means of a wire 50 passed through apertures in one or both of the nuts and through the adjusting rod 35, the ends of the wires being secured, for example, by a detachable lead seal 50a of well known type. This will preclude 'tampering with the device or any maladjustment of the parts which might adversely affect operation of the saw as a whole. Readjustment of the parts, however, may be made by any authorized and qualified individual after the seal is broken and the wire withdrawn.

In the operation of the saw, a considerable longitudinal vibration is set up in the member 25 and adjusting rod 35. This vibration, if uncontrolled, has a tendency to separate the elements of the ratchet device described above, to loosen the nuts 48 on the adjusting rod, and to turn the said rod itself in the threaded bore 34 of the member 25, with consequent maladjustment of the parts and modification of the normal chain tension. The Washer or spring 40 affords the necessary control of this vibration. It insures continuous firm contact between the elements of the ratchet device; exerts continuous pressure, through the sleeve 46a, upon the nuts and prevents loosening and displacement of the nuts on the adjusting rod; and, with the spring 31,. maintains the rod in the normal adjusted position with respect to the bore 34 in which the rod is threaded.

By reason of the tracking of the chain 5 in the rail 2 as previously described, it is essential, in order to reduce frictional wear to a minimum, to provide for adequate. lubrication between the chain and the rail. As previously described, the outer end of the rail is secured in the tensioning head by means of the screw 2! which passes through an opening 41 in the rail. The rail is provided with a corresponding opening 48 which is placed directly opposite to the opening 41, and the openings 41 and 48 are symmetrical with respect to the longitudinal center line of the rail 2 for a purpose hereinafter described. The filler plate I2 is recessed from its lower edge, as indicated at 49, Fig. 9, in registration with the openings 48 in the side plates 9 and II, and a corresponding recess 5| is provided at the upper edge of the filler plate l2 for registration with the side plate openings 41, as indicated in broken lines in Fig. 2. The side member I9 of the tensioning head 3 is provided with a threaded opening 52 in registration with the opening 48 in the rail, and into this opening 52 is threaded the terminal fitting 53 of a duct 54, which extends, as illustrated in Fig. 7, to the reservoir chamber I6 of the ten;- sioning head. Also as shown in Fig. '7, the duct 54 is provided with a suitable valve 55, so that v of the guide rail, and this opening receives a threaded dowel plug 51 of the form shown in Figs. 8 and 10, this plug having at its inner end a flattened extension 58, which in assembly and as shown in Fig. 8 projects through the opening 48 in the rail 2 and in conjunction with bolt 2| fixes securely the guide rail to the tensioning head. The flattened sides, or equivalent, in the extension 55, as shown in Fig. 8, afford a certain amount of clearance between the extension and the peripheral edges of the holes 48 in the outer plates 9 and H of the rail 2. The dowel plug 51 is provided with an axial bore 59, the outer end of which is threaded, as indicated at 5|, in Fig. 8. Projecting into the bore 59 is a rod 52, and this rod projects beyond the end of the plug 51 and into the terminal fitting 53 previously referred to. The rod 52 has a head 53, and immediately adjoining the head a threaded portion 54 which fits the threads 5! of the bore 59. When oil is allowed to pass through the duct 54, it comes in contact with the outer end of the rod 52 and follows the surface of the rod to the end of the plug 51. From this point, it passes through the flattened portions of the extremity 58 of the plug, and thus obtains access to the space 49 between the side plates 9 and II. From this space the oil passes into the recess 49, and therefrom into the slot i4, wherein it contacts with the portions of the saw chain which travel in the slot and provides for a thorough lubrication of the contacting surfaces of the saw chain and the guide rail 2. The passage of the lubricant to the chain slot is continuous and extends to the entire length of the track and of the guide rail.

The symmetrical location of the holes 41 and 48 in the rail 2 permits the position of the rail in the frame as a whole to be reversed. In this reversal, the clamping screw 2| passes through the hole 48, and the hole 41 then becomes the lubricating aperture, functioning with the lubricating elements as described above. It is preferable also that the means for securing the rail 2 in the head frame I correspond to the means for securing the rail in the tensioning head so that the position of the rail in the assembly may be reversed from end to end. The advantage of this device resides in the fact that in the use of the saw, the wear is not uniform in the rail, but occurs in certain localized areas, and the ability to install the rail in assembly in the various positions described provides for a longer useful life of this element.

As shown in Fig. 1, the base or head frame I is provided at its inner or forward end with an extension 55 and an adjoining boss 55. The extension 65 is provided with bosses for attachment of the inner end of the rail 2 and of the guard rod 22, and contains also the openings or passages which admit the toothed chain 5 to the interior of the gear case 5. Preferably the forward face 51 of the extension 55 lies at right angles to theplane of the rail 2 and of the chain 5, and the side face 58 of the extension in parallel relation to this plane. Similarly the forward face of the boss 55 will preferably occupy a position at right angles to the face 68 of the extension 85. In making a simple out such, for example, as in removing the end of the beam ll shown in Fig. 1, the face 51 of the extension 55 may be brought to bear against the side of the beam so as to hold the saw securely against the thrust imposed by the action of the cutting teeth upon the wood.

In other types of cut, the extension 55 and face 61 may also function to support the saw in 'board which may'beeecured timber" with the direction sired cut. with the boss 06 abutting iscutat ll shown in l'ig.1. For this purpose, as shown in Fig. 15, a suitably formed metal guide I! may be secured by means of attached spikes I. to the side of the pile. In this case, the forward face of the boss 8 bears against the outer side face of the guide bar II, and the face 88 of the extension 65 rests upon the upper edge of the side guide bar and keeps the cutting chain at a safe distance from the metal guide bar II, the said berthusactingasasupporting and guiding medium for the saw without injury to the cutting I claim:

1. In a chain saw, the combination with an endless toothed chain, of a support for said chain including relatively spaced rotary elements, and means for relatively adjusting said elements to tension the chain, said adjusting means comprising an adjusting element, a manual actuator for said adjusting element, a friction clutch operatively connecting said actuator with the adjusting element, a single resilient means exerting pressure tending to engage said clutch and operative to resiliently tension the said chain, and means for regulating the effective pressure of said resilient means.

4 mosa c desired position with to the work. to said take-up elemenhsaid ratchet affording! Inl'igs.12tol4,forexamplh,1haveillustratcd frictioncouplingforoperaiingtheelementintbo directiontotensionthechainandapodtivecoupling for operating the element in the opposite directiontorelievesaidtensionandaapringinterposed between the take-up element and the actuatonsaidspringreguiatingtheeflectivefrictioninsaidcouplingandbeingopentivsthrough said take-up element to resiliently tension the chain.

5.1nachainsaw,atcothedchain.apairof 2. In a chain saw, the combination with a toothed chain, of a pair of rotary elements supporting said chain, and take-up means operatively associated with one of said elements for tensioning said chain. saidtake-up means comprising a threaded take-up element, a manual actuator, and means for frictionally coupling said actuator with the said take-up element, said cou- I in the opposite direction to relieve said tension,

said coupling means including a friction-regulating resilient element operative through said takeup element to resiliently tension the chain.

4. In a chain saw, the combination with a toothed chain, of a pair of rotary elements supporting said chain, and take-up means operatively associated with one of said elements for tensioning said chain, said take-up means comprising a threaded take-up element, a manual actuator, ratchet means for coupling the actuator ing framework for said chain and elements, and

meansioradjustingoneoi!saiclelementsinthoframefortensioningsaidchaimsaidadjusting means comprisingabearingiorsaidelement ad- 'justably supported in the frame. a threaded adjusting element for said bearing, a manual actuator for said adjusting element seating against said frame, a sprin confined between said actuator and the adjusting element and operative through the latter to resiliently tension the chain, and friction means regulated by said spring for coupling said actuator to the said adjusting element.

6. In a chain saw, a toothed chain, a pair of rotary elements supporting said chain, a supporting framework for said chain and elements, and means for adjusting one of said elements in the frame for tensioning said chain, said adjusting means comprising a bearing for said element slidably supported in the frame, an adjusting element having threaded connection with said bearing, a manual actuator, a spring interposed between the actuator and said adjusting element and exerting pressure to frictionally couple said actuator to said element, and means affording a thrust bearing for said actuator on the frame whereby manipulation of said adjusting element through the actuator may operate through said spring to resiliently tension said chain 7. In a chain saw, a toothed chain, a pair of rotary elements supporting said chain, a supporting framework for said chain and elements, and means for adjusting one of said elements in the frame for tensioning said chain, said adjusting means comprising a bearing for said element slidably supported in the frame, an adjusting element having threaded connection with said hearing, a manual actuator seatin upon the frame, a friction clutch for operatively connecting the said actuator with the adjusting element, and a spring confined under pressure between said actuator and the adjusting element and operative both to resiliently tension the chain and to frictionally bind said actuator through said clutch to the adjusting element.

8. In a chain saw, a toothed chain, apair of rotary elements supporting said chain, a supporting framework for said chain and elements, and means for adjusting one of said elements in the frame for tensioning said chain, said adjusting means comprising a bearing for saidelement slidably supported in the frame, an adjusting element having t eaded connection with said bearing, a sleeve e bracing said adjusting element and seating in the frame, a spring confined under pressure between the said sleeve and the adjusting element and exerting pressure upon the sleeve to hold it to its seat and upon the said element in a direction to tension the chain, and means on said adjusting element and movable therewith for frictional engagement with said sleeve under pressure of said spring whereby rotation of the antacid alcove is tr i dtted irictionally to the adjusti element. a in a chain saw, a toothed chain, a pair of rotary elements supporting said chain, a supporting framework for said chain and elements, and means for adjusting one of said elements in the trains for tensioning cold chain, said adjusting nle comprising a ring for said element slidably supported in the i'rame, an adjusting element threaded connection with said bearing, a sleeve embracing said adjusting element and seating in the irame, a spring confined under more between the said sleeve and the adjust ina element and exerting pressure upon the sleeve to hold it to its seat and uponthe said element in a direction to ion the chain"; a collar onsaid adjusting element and movable therewith, said collar hot-ratchet engagement with said sleeve rotary elements supporting said chain, a supporting framework for said chain and elements, and means for adjusting one of said elements in the irame ior tensioning said chain, said adjusting means comprising a bearing for said element slidably supported in the frame, an adjusting element having threaded connection with said bearina, a sleeve embracing said adjusting element and seating in the frame, a spring confined under pressure between the said sleeve and the adjustina element and exerting pressiire upon the sleeve to hold it to its seat and upon the said element in a direction to tension the chain, means openatively connected with said adjusting element and initially held by said spring in frictional enaagernent with said sleeve for transmitting rotation oi the sleeve to the adjusting element, and supplementary resilient means independent of said spring ior maintaining positive engagement oi said transmitting means with the sleeve.

ii. In a chain saw, a toothed chain, a pair of notary elements supporting said chain, a supportina framework for said chain and elements, and means for adjusting one of said elements in the frame for tensioning said chain, said adjusting means comprisin a bearing for said element slidably supported in the frame, an adjusting element having threaded connection with said bearing, a sleeve embracing said adjusting element and seating in the frame, a spring confined under pressure between the said sleeve and the adjusting element and exerting pressure upon the sleeve to hold it to its seat and upon the said element in a direction to tension the chain, a collar carried by and movable longitudinally of said adjusting element for frictional engagement with said sleeve, means for limiting the movement of said collar longitudinally of the adjusting element whereby said spring is operative initially to enpage the collar with the sleeve, means for preventing'relative rotation oi said collar and the adjusting element whereby rotation of the sleeve may be transmitted through the collar to said element, and supplementary resilient means for maintaining the collar in frictional engagement with the sleeve.

12. In a chain saw, a toothed chain, a pair of rotary elements supporting said chain, a supportirameworlr ior said chain and elements, tor usting one oi said elements in t frame for oning said chain, said adjusting means comprising a bearing i'or said element slid ably supported in the frame, an adjusting ele ment having threaded connection with said hear ing, a sleeve embracing said adjusting element and seating in the frame, a spring confined under pressure between the said sleeve and the adjusting element and exerting pressure upon the sleeve to hold it to its seat and upon the said element in a direction to tension the c a collar carried by and movable longitudinally oi said adjustina element for frictional engagement with said sleeve, means ior preven relative rotation oi said collar and the adjusting element whereby rotation of the sleeve may be 1 we 1n tted through the collar to said element, a stop not on said artjusting element, ting the lotudinal move ment or the collar on the adjusting element whereby said spring is operative initially to en till gage the collar with the sleeve, and a spring in terposed between the said nut and the collar and exerting pressure to retain the collar in trio-- tional engagement with the sleeve.

13. In a chain saw, a toothed chain, a pair of rotary elements supporting said chain, a support-- ing framework for said chain and elements, and means for adjusting one oi said elements in the frame for tensioning said chain, said adjustina' means comprising a bearing for said element slidably supported in the trams, an adjusting olement having threaded connection with said hear ing, a sleeve embrac id adjusting element and seating in the frame, a spring contined under pressure between the said sleeve and the adjust" ing element and exerting pressure tmon the sleeve to hold it to its seat and upon the said element in a direction to tension the chain, a collar carried by and movable longitudinally oi said adjusting element for frictional engagement with said sleeve, means for preventing relative rotation oi said collarand the adjusting element whereby rotation of the sleeve may be transmitted through the collar to said elementra stop member on said adjusting element, limitina the movement of the collar lonaitully or the arliustina ele" inent whereby said spring is eilective initially to hold the collar in engagement with the sleeve, and resilient locking means engaging said collar and held by said stop member under partial com pression, said resilient locking means exerting pressure to maintain the collar in frictional engagement with the sleeve while permitting relative movement of said adjusting element in the longitudinal direction with respect to said sleeve and collar.

14. In a chain saw, a toothed chain, a pair oi rotary elements supporting said chain, a support ing i'rameworlr for said chain and elements, and means for adjusting one of said elements in the frame for tensionina said chain, said adjusting means comprising a bearing for said element slid ably supported in the frame, an adjusting screw having threaded connection with said hearing, a sleeve embracing said adjusting screw and seat-- ing in the frame, a spring confined under pres sure between the said sleeve and the adjusting screw and exerting pressure upon the sleeve to hold it to its seat and upon the said screw in a direction to tension the chain, a collar carried by and movable longitudinally of said adjusting .toothedehamutamii-oimtaryelementsmmpoi-ting said 'chain, a base member comprising amounting for one oi said elements, a guide plate for said chain secured at one end to said member. a tensioning head having a slot for receiving the other end of said guide plate and means for securing the plate in the slot, a, bearing for the other of said elements slidably mounted in the tensioning head, said head having a second slot in accurate alignment with the slot first named. a key in said second slot coactive with the said bearinz tor maintaining the associated element in accurate alignment with the said guide plate, and means ior adjusting said bearing in the head to tension the said chain.

ARTHUR RBI-UH. 

